Lidar scanning with expanded scan angle

ABSTRACT

In described examples of a system for outputting a patterned light beam, the system includes: an illumination source; a positive optical element positioned to receive light from the illumination source and to output converging light; a reflective element positioned to receive the converging light from the positive optical element, the reflective element configured to reflect the converging light to form a scan beam; and a negative optical element to receive the scan beam from the reflective element, the negative optical element configured to output the scan beam to a field of view.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional Application of U.S. patent applicationSer. No. 15/591,974, filed May 10, 2017, which claims the benefit ofpriority to U.S. Provisional Patent Application Ser. No. 62/334,810,filed May 11, 2016, entitled “DMD LIDAR Scanning Optics, and Correctionof DMD Aberrations Using Holographic Patterns,” which Applications arehereby incorporated herein by reference in their entireties.

TECHNICAL FIELD

This relates generally to light detection and ranging (lidar) systems,and more particularly to lidar systems having a scan beam with anexpanded scan angle.

BACKGROUND

The term “lidar” or “LIDAR” is a portmanteau of the words “light” and“radar” to describe systems using light for ranging and depth imagingsystems. More recently, the term lidar is an acronym for “LightDetection and Ranging.” In lidar systems, a source transmits light intoa field of view, and the light reflects from objects. Sensors receivethe reflected light. In some lidar systems, a flash of light illuminatesan entire scene. In the flash lidar systems, arrays of time-gatedphotodetectors receive reflections from objects illuminated by thelight, and the time it takes for the reflections to arrive at varioussensors in the array is determined. In an alternative approach, a scanbeam (such as a raster scan beam) can illuminate a scene in a field ofview (FOV) in a continuous scan fashion. A source transmits light orlight pulses during the scan. Sensors that can also scan the scan beampattern, or fixed sensors directed towards the FOV, receive reflectivepulses from objects illuminated by the light. The light can be a scannedbeam or a moving spot. Time-of-flight computations can determine thedistance from the transmitter to objects that reflect the light from thescan beam. The time-of-flight computations can create distance and depthmaps. The depth maps are displayed. Light scanning and lidar are used ina variety of applications, including: ranging; metrology; mapping;surveying; navigation; microscopy; spectroscopy; object scanning; andindustrial applications. Recently, applications also include security,robotics, industrial automation, and mobile systems. Vehicles use lidarfor navigation and collision avoidance systems. Autonomous vehicles andmobile robots use lidar for motion control and collision avoidance.

To scan objects in a FOV, a useful lidar scan beam traverses a largescan angle from the transmitter to the scene, such as between 30 and 100degrees, or more.

SUMMARY

In described examples of a system for outputting a patterned light beam,the system includes: an illumination source; a positive optical elementpositioned to receive light from the illumination source and to outputconverging light; a reflective element positioned to receive theconverging light from the positive optical element, the reflectiveelement configured to reflect the converging light to form a scan beam;and a negative optical element to receive the scan beam from thereflective element, the negative optical element configured to outputthe scan beam to a field of view.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a lidar system.

FIG. 2 illustrates a lidar system in a vehicular application.

FIG. 3 illustrates a raster scan pattern.

FIG. 4 illustrates a projection system using a DMD.

FIGS. 5A and 5B illustrate, in two graphs, a beam radius plotted againstdistance from a transmitter.

FIG. 6 illustrates a lidar system arrangement.

FIG. 7 illustrates another lidar system arrangement.

FIG. 8 illustrates a lidar system arrangement using a digitalmicromirror device (DMD).

FIG. 9 illustrates distortion of light reflected from a DMD.

FIG. 10 illustrates an example of a DMD used to reflect light.

FIG. 11 illustrates an example of a DMD used to reflect light withcorrection applied to the DMD.

FIG. 12 illustrates an example of a DMD used to display a diffractionpattern.

FIGS. 13A, 13B and 13C illustrate a diffraction pattern andcorresponding images.

FIG. 14 is a block diagram illustrating one algorithm for generating aDMD binary, amplitude-only diffraction pattern.

FIG. 15 is a block diagram of an arrangement for a lidar system.

FIG. 16 illustrates another arrangement for a lidar system.

FIG. 17 is a flow diagram of an example method for use with examplearrangements.

FIG. 18 is a flow diagram of a method of generating correcteddiffraction patterns.

FIG. 19 is a flow diagram of a method of forming corrected diffractionpattern templates.

FIG. 20 is a flow diagram of a method of using stored correcteddiffraction patterns to form a scan pattern.

DETAILED DESCRIPTION

In the drawings, corresponding numerals and symbols generally refer tocorresponding parts, unless otherwise indicated. The drawings are notnecessarily drawn to scale.

The term “coupled” may also include connections made with interveningelements, and additional elements and various connections may existbetween any elements that are “coupled.”

In mechanically scanned lidar systems, a rotating mirror or mirrors cancause a beam to scan the scene in the field of view. Sensors detectlight reflected from objects in the field of view by backscattering. Thefixed scan patterns result from mechanically rotating a laser or frommechanically rotating a mirror reflecting light from a laser orreflecting light received from a collimating lens fed by a laser. Thesemechanical systems include a variety of mechanical components (such asmotors, gears, rotors, and moving mirrors) that have substantial powerand weight requirements, require maintenance, are subject to failure,and may require repair.

Lidar is useful in autonomous vehicles. For autonomous vehicleapplications, commercially available lidar systems include manycomponents and moving parts, such as mechanical motors, rotators andhousings arranged for mounting the system on vehicle roofs. An examplevehicular lidar system includes eight assemblies of eight lasers, eachto form a sixty-four laser/detector assembly mounted on a vehiclerooftop. The lasers and detectors mount in a rotating housing thatrotates at up to 20 Hz. Motors and rotating mechanical parts provide thehigh-speed rotation. Each of the eight assemblies includes multiplelasers and detectors. Examples of these systems are commerciallyavailable from Velodyne LiDAR, Inc. Such systems are relatively high incost, are mechanically and electrically complex, require special powerand maintenance, require substantial processing, and are physicallylarge, so they affect the appearance and exterior surfaces of thevehicle.

In example arrangements herein, ranging and/or depth measurement lightdetection systems use scan beams with scan angles of greater than 30degrees. In an example arrangement, an analog mirror or analog MEMSdevice with a reflective surface is a reflective element with opticsarranged to achieve an expanded scan angle. In another arrangement, adigital micromirror device (DMD) is a reflective element with opticsarranged to expand the scan angle. In some arrangements, the DMDsdisplay diffraction patterns illuminated to provide scan beam patterns.In a further arrangement, the DMD modifies an optical wavefront bydisplaying holographic patterns. The use of the DMD to reflect light andmodify a wavefront enables constructive interference to form a spot orpattern at an arbitrary distance away from the transmitter. By varyingthe patterns displayed by the DMD, the spot or pattern can scan thefield of view in a raster scan or other scan pattern. Because the systemcan modify patterns displayed on the DMD, the patterns can be modifiedto compensate for characteristics in the optics and to compensate forinherent distortion in the DMD device itself. Further, in examplearrangements, the beam radius can be more collimated and free fromdistortion across the expanded scan angles.

In another arrangement, a phase spatial light modulator (PSLM) is areflective element to change phase of incoming light and input a beam tooptics to achieve a scan beam with an expanded scan angle. In a furtherarrangement, a liquid crystal on silicon (LCoS) device is a reflectiveelement that modulates phase of incoming light rays to form a scan beamand input the scan beam into optics to achieve the expanded scan angle.

In lidar systems including the lidar systems of example arrangements,detectors sense the scan pattern light reflected from objects in theFOV. Time-of-flight calculations determine distance or depthinformation. In example arrangements, diffractive patterns display on aDMD arranged as a reflective element. At least one coherent light sourceilluminates the DMD. In one arrangement, the coherent light source is alaser. In another alternative arrangement, the coherent light source isa near infrared laser. In still another alternative arrangement, thelight is pulsed. At a predetermined distance in the field of view of thesystem, an image such as a pattern, a spot or multiple spots or beamsform by interference of the light waves traveling from the DMD. Sensorsdetect reflections from the scan patterns that occur due tobackscattering from objects. Distance to the objects can be determinedusing time-of-flight computations. In example arrangements, a number ofiterative or non-iterative algorithms generate the diffractive images.In some of the algorithms, Fourier transforms simulate the desired farfield image based on the DMD binary pattern. In an example, inverseFourier transforms compute the diffraction patterns that are appropriateto form desired scanning patterns in the far field. After computing thecomplex inverse Fourier transform data, the algorithms perform filteringof the data and quantizing of the complex inverse Fourier transformdata. In this manner, example arrangements create binary hologram imagesfor display on the DMD.

In another example, a plurality of two dimensional diffraction patterntemplates needed to form a desired scanning pattern are stored forretrieval. In one example, the patterns include raster scanningpatterns. In scanning the field of view, the diffractive patterntemplates display on the DMD in sequences designed to create the desiredscanning pattern. In alternative examples, real time computing computesthe two dimensional DMD diffraction pattern data as needed. The realtime computation outputs video data to display a scanning sequence usingdiffractive patterns on the digital micromirror device. In a sceneadaptive example, the system resolution increases in the scanningpattern for a selected portion of the field of view with an object ofinterest. Using coarse resolution over a part of the scene with finerresolution for an area of interest in the scene improves performance. Byleaving a portion of the scan pattern in a coarse resolution, processingtime improves. By increasing resolution in an area of interest, systemperformance and resolution improves. In these arrangements, theresolution is defined by the number of individual beam positions coveredduring the scan or the divergence of the beams themselves.

Further, in some example arrangements, the holographic patterns aremodified using field collected test data or optical ray simulations tocompensate for distortion in the optical elements, and/or to compensatefor beam distortion caused by the reflective element. For example, whena DMD is a reflective element, geometric distortion occurs due to thephysical characteristics of the array of micromirrors and the spacingbetween the mirrors. In some arrangements, the diffraction patterns aremodified to compensate and to reduce or eliminate the geometricdistortion in the scan beam. In some arrangements, the modifications tothe diffraction patterns is a predistortion applied before display ofthe diffraction patterns, in order to compensate for known distortion oraberration in the system.

FIG. 1 is a block diagram of a lidar system 100. In FIG. 1, system 100includes a laser (or other light source) 101 arranged to illuminate amirror 103. A rotating mount 106 rotates mirror 103 so that the laserbeam movably scans across the field of view. In FIG. 1, a human FIG. 105is in one part of the field of view, and a tree 113 is in another partof the field of view. The tree 113 and the human FIG. 105 are located atdifferent distances from mirror 103. In FIG. 1, the light is shownpulsed.

When a pulse of laser energy enters the field of view from the surfaceof mirror 103, reflective pulses appear when the laser light illuminatesan object in the field of view. These reflective pulses arrive at mirror109. Mirror 109 can also movably rotate on a rotating mount 108. Thereflective pulses reflect into a photodetector 111. The photodetector111 can be any of a number of photodetector types; including avalanchephotodiodes (APDs), photocells, and/or other photodiode devices. Imagingsensors such as charge-coupled devices (CCDs) can be the photodetectors.

As shown in FIG. 1, the photodetector 111 receives reflective lightpulses. Because the time the transmit pulses are transmitted from laser101 onto mirror 103 is known, and because the light travels at a knownspeed, a time-of-flight computation can determine the distance ofobjects from the photodetector. A depth map can plot the distanceinformation.

FIG. 2 is a block diagram of an example vehicle mounted lidar system200, such as in autonomous vehicle applications. In FIG. 2, a car 201includes a mechanically rotating lidar system 203 mounted on the rooftopof the vehicle. The rotating lidar system transmits laser pulses andmeasures corresponding reflections from objects around the system. Usingtime-of-flight calculations for the reflections based on the speed oflight, the system can compute distances. Lidar systems for autonomousvehicles are available from Velodyne LiDAR, Inc. An example system hassixty-four lasers arranged with corresponding detectors mounted in arotating housing with a rotator motor that rotates the housing at up to20 Hz. This system requires power to the motor, the many lasers, and themany detectors, and it requires substantial physical space on thevehicle's roof.

FIG. 3 depicts in a simple diagram a representative pattern for scanninga scene using light. In the example of FIG. 3, a “raster” pattern shownby line 301 proceeds horizontally along a line position from one side ofa field of view to the opposing side. The raster pattern returns to scanacross the scene at another horizontal position vertically displacedfrom the first row. The pattern 300 shown in FIG. 3 illustrates a commonscanning pattern for a single spot or beam. However, the variousarrangements can use any number of alternative scanning patterns.

An important technology for light processing is DLP® technology fromTexas Instruments Incorporated (“TI”). TI DLP® home and cinemaprojectors, televisions, and sensors are in widespread use. Thesesystems use one or more spatial light modulators having digitalmicromirror devices (DMDs), a reflective spatial light modulatortechnology developed by Texas Instruments Incorporated. In a DMD, a twodimensional array of mirrors is formed. Each mirror is over a hingedtorsional tilting mechanism. Electrical signals control the torsion totilt the mirrors. The mirrors each have a corresponding data storageunit that are each individually addressable, and each micromirror can beswitched between two states many thousands of times per second. DMDdevices available from Texas Instruments Incorporated can include manythousands and even millions of the micromirrors and can support variousvideo resolutions. DMDs are reliable and robust especially as adiffractive beam scanner, because even if substantial portions of thearray of micromirrors become inoperable, the high number and small sizeof the digital micromirrors achieve inherent redundancy. DMDs haveproven to be highly reliable, long life, solid-state devices forprocessing light. Because a DMD reflects light and each micromirror hasan ON state and an OFF state, the DMD acts as a binary amplitudemodulator.

FIG. 4 depicts in a simplified block diagram a DMD used as a spatiallight modulator (SLM) in an image projector system. In system 400, asingle light source 407 and illumination optics 409 direct light fromthe light source onto the face of a DMD 401. Some systems use multiplelight sources such as red, green and blue light sources. Some colorlight systems use color wheels to create colored light from a singlelight source. Some systems use multiple DMDs. DMDs such as DMD 401 aremanufactured using micro-electromechanical system (MEMS) technologybased in part on semiconductor device processing. An array ofmicromirrors 403 is disposed over a semiconductor substrate 405. In anexample, the micromirrors include aluminum faces and are each mounted ona hinged torsion mechanism. The micromirrors 403 attach to a torsionhinge and can be tilted using electronic signals. The signals areapplied to electrodes for each micromirror that control a tilt byapplying torsion to pivot the micromirrors about an axis. In an exampleDMD device, a two dimensional array of thousands or perhaps millions ofthe micromirrors form a WGA, XGA, 720p, 1080p or higher resolutionimaging device. The micromirrors 403 reflect illumination light from theillumination optics 409 to a projection lens 406. A beam of lightprojects from the system 400. By displaying an image using the DMD andilluminating the DMD micromirrors, the reflected beam of light caninclude an image for display on a surface such as a screen or wall. Themicromirrors 403 are individually addressable, and each has anassociated storage memory cell that determines the state of themicromirror during an active illumination period.

The micromirrors 403 each have two addressable states: a first “ON”state; and a second “OFF” state. In the ON state, the micromirrors 403in FIG. 4 tilt in a first position away from a FLAT position, which inthe plan view of FIG. 4 is aligned vertically. The FLAT position is theposition a mirror takes when it is unpowered. The tilt occurs due tosignals on an electrode that cause the torsion hinges to flex. In system400, the micromirrors 403 in the ON state reflect incoming light fromillumination optics 409 outwards to the projection lens 406. In the OFFstate, the micromirrors 403 tilt to a different position. In thisexample arrangement, mirrors in the OFF state reflect the light awayfrom the projection lens 406. In some arrangements, the OFF state lightreflects to a “light dump” (not shown) or thermal energy collector. Byvarying the tilt positions using electrical control signals, each of themicromirrors 403 can direct reflected light to the projection lens 406.In at least one example, the FLAT position is 0 degrees, and a DMD fromTexas Instruments Incorporated has an ON state tilt of about +12 degreesand an OFF state tilt of about −12 degrees. Other DMD devices providedifferent tilt angles, such as +/−10 degrees, or +/−17 degrees.

This type of DMD is commercially available and sold by Texas InstrumentsIncorporated. For example, the Texas Instruments Incorporated deviceDLP3000 has an array of 608×684 micrometer sized mirrors, equating tomore than 400,000 micromirrors. The DLP3000 is one example DMD but manydifferent DMD devices are available from Texas Instruments Incorporated.

In projection systems using a DMD, illumination optics provide a cone oflight onto the DMD while the mirrors tilt “on” or “off” to provide thedisplayed image. Projection optics then focus that image onto a surfacefor display. Various display systems use DMDs as projectors.Applications include: theatre and conference room projectors thatdisplay on a wall or screen; rear projection televisions that projectonto a display screen; home projectors; sales or presentationprojectors; hand held and pico-projectors; heads up displays foraviation, marine and automotive applications; virtual reality andwearable goggles; personal video players displays; and smart glassesdisplays. Each of these applications uses the DMD as a reflectivespatial light modulator.

In another arrangement, the two-dimensional array of micromirrors in aDMD can act as a diffraction grating. The pitch (the spacing between themirrors) of the DMD devices varies from a few microns to about fourteenmicrons. The pitch of these devices is small enough, when compared tothe wavelengths of coherent light sources such as lasers, to exhibitsignificant diffraction. In projection systems, the diffraction effectsare not desirable, and these effects are minimized by optical design.

In certain example arrangements, a DMD is a useful reflective element ina lidar scanning beam. However, without special optics, the relativelysmall active area of a DMD results in a small exit beam size. A smallbeam size at the transmitter results in an increasing beam divergenceangle in the far field, which is less desirable than a small beamdivergence angle in a scanning application.

FIGS. 5A and 5B illustrate the relationship between the beam radius atthe exit of a beam transmitter and scan beam divergence at a distancefrom the transmitter. In FIG. 5A, for a beam of 905 nanometer wavelengthwith a beam quality factor M{circumflex over ( )}2 of 35, the beamradius is plotted against the distance from the exit of the transmitter.The curve labeled “5.3 millimeter” shows the results for a beam having aradius of 5.3 millimeters at the exit (the origin 0 on the horizontalaxis.) As the distance from the origin increases, the beam radiusplotted on the vertical axis also increases. For the beam of 5.3millimeters, at a distance of 5 meters, the beam radius is slightlygreater than 5 millimeters. In FIG. 5A, a second beam having a smallerradius at the exit of the transmitter is plotted. The curve labeled “2.3millimeters” shows a beam having a radius of 2.3 millimeters at theorigin on the horizontal axis shows that at 5 meters from the origin,the beam radius is over 20 millimeters. The curves in FIG. 5A illustratethat a smaller beam radius at the origin results in a beam with widerbeam divergence as the distance from the transmitter increases. A beamwith larger diameter at the exit of the transmitter has lower beamdivergence in the far field at a distance from the transmitter.Conversely, a beam with small diameter at the exit of the transmitterhas greater divergence in the far field at a distance from thetransmitter. For a beam used to scan a field of view such as in a lidarsystem, a smaller beam divergence is desired.

FIG. 5B further illustrates the point. In FIG. 5B, the beam radius forthe same two beams is shown at increasing distances from the transmitterexit, at the origin or “0” on the horizontal axis. As the distanceincreases up to 35 meters on the horizontal axis, the beam radius alsoincreases. For a beam of 5.3 millimeters at the origin, the curvelabeled “5.3 millimeters” shows that at a distance of 35 meters from thetransmitter exit, the beam radius has increased to about 60 millimeters.In comparison, for the beam having a radius of 2.3 millimeters at theorigin on the horizontal axis, the curve labeled “2.3 millimeters” showsthat a distance of about 25 meters from the transmitter exit, the beamradius has already reached 100 millimeters. Thus, the beam divergencefor a smaller initial beam radius is much greater than for a beam havinglarger initial beam radius.

As described hereinabove, in some arrangements a DMD is a reflectiveelement in a lidar transmitter. In a lidar scan beam transmitter where aDMD device is used as a reflective element without the use of optics,the scan angle that can be achieved is limited to an angle ofarcsin(wavelength/pixel pitch). In an example, using light with awavelength light of 905 nanometers while using a DMD device with a pitchof about 7.6 microns, the scan angle is limited to about 6.8 degrees.This scan angle needs to increase by a factor of about 5-15× or more toachieve a useful scan angle for a lidar system. The scan angle needs tobe between 30 to 100 degrees, or more.

FIG. 6 is a block diagram in a plan view for an example arrangementusing a “reverse telescope” technique to expand a scan angle in a lidarsystem using a reflective element. In the system 600 of FIG. 6, anegative optical element, here shown implemented using a lens group 601,is arranged with an illumination source 603, which can be a laserincluding a near infrared laser source and optics to condition theoutput of the light source and focus light rays onto the reflectiveelement 607. In this example, the reflective element 607 is implementedusing a DMD device, and the negative optical element 601 can be part ofa reverse refractive telescope. In FIG. 6, the scan beam is shown in raytracings at three different scan positions 611, 613, 615 to illustratethe total scan beam angle that can be achieved. Additional positiveoptical elements 609 focus and converge the output rays leaving thereflective element 607 into the input of the negative lens group 601. Inan example, the optical elements 609 are positive, converging the lightrays from the reflective element 607, and the lens group 601 isnegative, expanding the angle that can be covered by the light rays ofthe scan beam in the FOV. In an example, the positive optical element609 and the lens group 601 taken together form a reverse refractivetelescope. In another example, the positive optical element 609 and thelens group 601 taken together form an afocal lens. The total scan angleθ is indicated by the beam positions 611 and 615. In an arrangement, thescan angle is greater than about 30 degrees, and in another arrangement,is between 30 and 100 degrees.

The arrangement in FIG. 6 increases the scan beam angle θ over thepossible scan angle obtained for a transmit beam employing a DMD as thereflective element without special optics. However, the beam diameter atthe DMD 607 exit is comparatively small, as described hereinabove withrespect to FIGS. 5A and 5B, increases the beam divergence in the fieldof view.

FIG. 7 is a block diagram of an additional example arrangement for abeam scanning system with an expanded scan angle. System 700 is shownwith a reflective element 707 arranged with a negative optical element701. In FIG. 7, similar reference labels are used for similar elementsin FIG. 6, for ease of understanding. For example, the negative opticalelement 701 corresponds to the negative optical element 601 in FIG. 6.An illumination source 703 such as a laser or a near infrared laser isshown emitting rays to a collimator 705. In the arrangement of FIG. 7,the reflective element 701 is positioned between a positive opticalelement 706 that converges the rays onto the face of the reflectiveelement, and the negative optical element 701, in contrast to thearrangement in FIG. 6 where the reflective element is outside theoptical path. Taken together, the positive optical element and thenegative optical element form a reverse telescope or a reverse afocallens. The collimated rays are focused by optical element 706 to convergeon the surface of reflective element 707. Reflected rays from thesurface of the reflective element 707 are input into the negativeoptical element 701, here shown as a part of a reverse refractivetelescope. The scan beam is shown in five different positions toillustrate the scan angle θ that is obtained. Position 711 is theleftmost position in this top view of system 700, the beam can then scanthrough position 713 to the center position 715, through position 717 tothe rightmost position 719. In an example, the scan angle θ from left toright can exceed 30 degrees. In a further example, the scan angle θ canexceed 50 degrees. In another example, the scan angle θ obtained by thearrangement in FIG. 7 can be between 30 and 100 degrees. In additionalexamples, the scan angle θ is greater than 45 degrees. Examplearrangements achieve an increased scan angle compared to arrangementswithout the negative optical element arranged with the reflectiveelement.

In contrast to the arrangement in FIG. 6, in system 700 the reflectiveelement 707 is positioned within the optical path of elements 706 and701, with the light rays focused by a positive optical element 706 toconverge on the reflective surface of reflective element 707. Thisarrangement enables a larger beam diameter leaving the reflectiveelement 707. As described hereinabove, the larger beam diameter at thetransmitter results in less beam divergence in the far field. The raytracings of the scan beam at positions 711, 713, 715, 717, and 719 inFIG. 7 illustrate less beam divergence than the corresponding beampositions shown in FIG. 6.

Reflective element 707 is, in one example arrangement, an analog MEMSmirror device. Analog MEMS mirror devices are commercially available.The analog MEMS mirrors rotate on two axes and so can obtain a varietyof tilt and rotate positions. By directing coherent light onto thesurface of the analog MEMS mirror, the rotate and tilt motion of theanalog MEMS mirror allows a scan beam to be generated that traverses apattern. The analog MEMS mirror device is fabricated using a processsimilar to semiconductor processing technologies. The reflective mirrorsurface can be a metal such as gold or aluminum. The analog MEMS mirroris a solid-state device, in that input electrical signals (not shown inFIG. 7 for simplicity) cause the MEMS mirror to rotate and tilt, noadditional mechanical systems, gears, rotors or motors are needed.Unlike a DMD MEMS device in which the micromirrors are placed in eitherone or another of two discrete tilt positions “ON” and “OFF” asdescribed hereinabove, the analog MEMS device mirror can take a varietyof positions obtained by applying a range of voltages to electrodes thatcause the mirror to move. The mirror moves on two axes so a reflectedbeam can scan a scene in front of a transmitter by rotating and tiltingthe MEMS mirror to direct the reflected scan beam.

As shown in FIG. 7, the scan angle θ of the beam that is obtained isquite large, as shown by the range from the beam in one position at 711to the opposing beam position at 719. Because the incident light isconverged before the reflective element 707, the exit beam obtained islarger in diameter than for the arrangement in FIG. 6. Also, asdescribed hereinabove, the larger diameter exit beam achieves smallerbeam divergence in the far field. Accordingly, a scan beam with smallerbeam radius in the FOV, and with an expanded scan angle, is achievable.

In the arrangement of FIG. 7, a scan beam has an increased scan angle,but some applications may benefit from using a DMD as the reflectiveelement 707. Use of a DMD device allows for additional compensation dueto the ability to display arbitrary patterns on the DMD, as furtherdescribed hereinbelow.

FIG. 8 is a block diagram of an alternative arrangement, where a DMD isthe reflective element in a lidar transmit system 800 using a portion ofa reverse refractive telescope to expand the scan angle. The arrangementof FIG. 8 is otherwise similar to the arrangement of FIG. 7. In FIG. 8,lidar system 800 includes a DMD 807 that reflects incident light to anegative optical element 801, in this example negative optical element801 is a lens group shown as a part of a reverse refractive telescope. Alight source 803 that can be a laser or a near infrared laser emitslight rays to a collimator 805. The rays from 805 enter optics 808 thatconverge the light onto the reflective surfaces of the micromirrors inDMD 807. The lens group 801 is a negative optical element that increasesthe scan angle. In alternative arrangements, other negative opticalelements replace the portion of the reverse refractive telescope. In oneexample arrangement, afocal lenses can form the negative optical element801.

In FIG. 8, the scan beam is in a range of positions 811, 813, 815, 817and 819 to illustrate the expanded scan angle obtained by use of thenegative lens group 801 to expand the beam angle.

As shown in FIG. 8, the arrangement provides an expanded scan angle θ.However, as shown by the light ray tracings in FIG. 8, the beam hasdiverged for reflected light that strikes certain portions of the DMD.For example, at beam position 811, the beam shows diverging rays in thelight ray tracing, because the diameter of the beam at position 811 isdifferent from the beam at position 819.

In an alternative arrangement, a phase spatial light modulator (PSLM) isthe reflective element. For example, in FIG. 8, a PSLM can be thereflective element 807 in FIG. 8 to form another example arrangement. APSLM is a device having an array of cells, with each cell imparting adifferent optical phase delay according to the electrical signal appliedto each cell. A PSLM device can have a liquid crystal device (LC), aliquid crystal on silicon device (LCoS) or a microelectromechanicalsystem (MEMS) device. A MEMS PSLM usually has an array of micromirrorsthat displace in a direction normal to the array plane in response toelectrical signals. The function of a PSLM is to change the shape of theoptical wavefront incident on the device. The PSLM can impart a linearphase delay on a wavefront thus steering the beam in a differentdirection. A PSLM can also impart a curved wavefront that can focus thewavefront in a manner similar to a lens. The primary advantage of a PSLMis that it can be quickly reconfigured to steer or focus a beam to adesired direction or focus to a desired plane.

The optical function of a PSLM in a lidar transmitter is different fromthat of a DMD described hereinabove. When using the DMD, light from thelight source is directed onto the DMD array and various pixels areturned on or off. Particular areas or points of interest within thescene can be selected by imposing a spatial linear phase pattern on thePSLM similar to a blazed diffraction grating such that the beam issteered. As a consequence, the light not in the region of interest isdirected to an area away from the output optics. In this manner, thePSLM can perform a similar function to the DMD in directing laser lighttoward the optics and out of the transmitter to steer a scan beam.

A linear phase function displayed on the PSLM directs the laser light ina desired direction. The phase front is altered for each beam direction,causing the beam to scan in a particular pattern required to obtainrange or reflectivity image of the scene in the FOV. Furthermore, bydisplaying a curved phase function on the PSLM, the beam can be focusedat an input to the optics.

In yet another example, the reflective element in FIG. 8 is a liquidcrystal on silicon (LCoS) device that forms an additional arrangement.The LCoS device can modulate the phase of light received from a lightsource and can output reflected light rays to the negative opticalelements to steer a scan beam. The use of the negative optical elementsexpands the angle the scan beam traverses in the field of view asdescribed hereinabove.

A DMD is a useful reflective element for example arrangements, when theDMD is combined with the expansion optics, but aberrations can occur inthe scan beam. FIG. 9 illustrates geometric distortion in lightreflected from a DMD device through the expanding optics. In FIG. 9, thedots represent the pattern obtained from reflection of light from thescanning DMD system. Even if the DMD diffraction patterns are generatedso that the beams are equally spaced in the far field, the opticalexpansion will result in some distortion of the positions in the farfield.

In the center portion of DMD 900 in FIG. 9, the dots representingreflected light align with the gridlines, indicating no distortion. Inthe regions 901, 903, 905, 907, where the dots represent light reflectedfrom corner regions of the DMD, the dots no longer align with thegridlines, indicating distortion. Because the distortion and the amountof distortion vary according to the location on the DMD that is used,the distortion is referred to as “geometric distortion.”

Geometric distortion can be corrected by generating DMD diffractionpatterns that pre-distort the beam positions such that they fall on therectangular grid. This is one form of distortion that can be correctedby the design of the diffractive patterns. In example arrangement,pre-distortion is applied to the diffractive patterns to compensate thedistortion.

The DMD diffractive patterns can also correct another form of opticalaberration created by the DMD. FIG. 10 is a block diagram of a system1000 with a DMD reflecting a converging beam. A light source andcollimator (not shown for simplicity) directs light rays into theoptical element 1008 that converges the light rays onto the surface ofDMD 1007, the DMD having an array of micromirrors. The resulting beam isshown in ray tracings in several beam positions including 1017, 1019after being reflected by DMD 1007. Region 1011 illustrates the resultand the distortion. Because the DMD acts like a diffraction grating inthe scan direction, the converging rays will be focused by differentamounts according to the diffraction order and scan angle. The differentfocus amounts result in beam astigmatism. In example arrangements, theastigmatism can be corrected in the generation of the diffractivepatterns.

FIG. 11 is a block diagram of a system 1100 similar to system 1000,where correction applied to the DMD corrects for the astigmatism in theDMD. The reference labels in FIG. 11 for elements similar to those inFIG. 10 are similar, for ease of understanding. For example, DMD 1107corresponds to DMD 1007 in FIG. 10. In FIG. 11, an optical element 1108converges light rays from a light source (not shown) onto the surface ofDMD 1107. The reflected beam is shown in ray tracings at severalpositions including 1117, 1119. Area 1111 shows the effects ofcorrection for aberrations in the DMD. The focal point of the beams fordifferent scan beam positions is at the same distance from the DMD 1107as shown in area 1111. This is in contrast to the astigmatism effectsshown in region 1011 of FIG. 10, where no correction is applied. Thecorrection changes the pattern displayed on the DMD 1107 to compensatefor distortion. By applying a correction that acts as a focusingelement, the beams can be correctly collimated for all scan angles.

When illuminated with a coherent light source the DMD can display binarypatterns that behave as binary holograms. These binary hologramsdiffract the light and can produce a variety of image patterns in thefar field. Diffraction changes the direction and distribution of lightdue to traversing apertures such as an opening or slit. On a DMD device,the individual micromirrors and the spaces between the individual DMDmirrors provide a natural diffraction grating. Further, diffractivepatterns displayed using the DMD can create desired patterns in a farfield image plane. These patterns result from interference (constructiveand destructive) between wavefronts of light traveling away from thediffractive pattern displayed on the DMD. The image patterns in the farfield are almost unlimited in variety. A single DMD and a singleillumination source can form many patterns in the far field image plane.

Co-owned U.S. patent application Ser. No. 15/202,315, filed Jul. 5,2016, entitled “Methods and Apparatus for LIDAR with DMD,” which ishereby incorporated by reference herein in its entirety, discloses theuse of diffraction patterns with a DMD to provide scan patterns forlidar systems.

To illustrate the diffractive characteristic of a DMD, FIG. 12 is ablock diagram of a far field image result obtained by illuminating anentire DMD micromirror array with a laser illumination source. In FIG.12, a system 1200 operates by directing the output of a laser 1205 ontothe mirror surfaces of DMD 1207. In this example, afocal lenses 1221 and1223 collect the output light and provide an afocal lens correction toilluminate a larger field of view than can be illuminated in a “lensless” system. However, the use of the afocal lenses 1221 and 1223 is notrequired, and in alternative arrangements, the system can be “lensless.” The far field image shown as 1209 in FIG. 12 is a pattern ofspots with a brighter spot in the center, and the pattern of spotssymmetrically surrounds the center spot. The pattern 1209 illustratesthat the DMD is acting as a diffraction grating for the illuminatinglaser. In example arrangement, the DMD displays an image containing aseries of black and white stripes. The pattern in the far field hasspots due to the diffractive properties of the DMD.

An example system can use the diffraction properties of the laser andthe DMD to form arbitrary patterns. For example, a pattern of lines canform as shown in FIG. 12. Further, by displaying a particulardiffraction pattern using the DMD, the system can create dots or otherpatterns at any arbitrary position in the field of view of the system.Because the DMD can rapidly switch between different diffractivepatterns, a sequence of patterns can display using the DMD, with eachdiffraction pattern illuminated by the laser. By continuing to displaydifferent patterns in the sequence, a scanning beam pattern or movingspot pattern forms at the far field. In this example system, the DMD isacting as a hologram display. The hologram pattern can cause a beam orbeams. The beams can result in reflections from objects located atdistances from a few centimeters to one hundred meters or even up toseveral hundred meters from the DMD and illumination source. In a lidarapplication, a receiver receives the reflections and time of flightcalculations determine depth information. The depth information can bevisually displayed to an operator or can be used in navigation,collision avoidance and guidance lidar systems.

Because the patterns displayed on the DMD are holograms or diffractionpatterns that result in images in the far field, focused opticalelements are not required. However, as described hereinabove, examplearrangements can include negative optical elements such as portions of areverse refractive telescope or other negative optical elements toexpand the scan angle covered by the scan beam pattern in the field ofview. Further positive optical elements placed between the illuminationsource and the DMD converge the light onto the surface of the DMD,resulting in an increased beam diameter in the reflected beam withcorresponding lower beam divergence in the field of view. A rapidlypulsing laser can illuminate the DMD. The laser can be at low averagepower levels and with short pulse durations that are “eye safe” so thata viewer will not suffer eye damage if the laser light strikes anobserver's eye. The system can use infrared, near infrared and otherillumination wavelengths.

An example system using a DMD as a diffraction pattern generator orhologram display device is described in a paper entitled “Digital micromirror device as a diffractive reconfigurable optical switch fortelecommunication,” by Blanche et. al., Journal ofMicro/Nanolithography, MEMS and MOEMS, Vol. 13 (1), January-March 2014,pp. 011104-01 through 011104-05 (hereinafter, “Blanche et. al.”), whichis hereby incorporated by reference in its entirety. In Blanche et. al.,the authors demonstrate that a diffraction pattern on a DMD can producean image at a desired point in an image plane. In an example systemdescribed in Blanche et. al., spot patterns are input as data to opticalfibers in an optical switch.

FIG. 13A shows an example desired pattern (described in Blanche et. al.)of a logo of Texas Instruments Incorporated, known as the “TI bug.” InFIG. 13B the DMD pattern needed for producing the image by diffractionis illustrated as it would be displayed using a two dimensional DMDarray. The pattern in FIG. 13B is not a recognizable image of the logoin FIG. 13A, FIG. 13B shows that the DMD is not projecting images as inan image projection system. FIG. 13C shows the resulting holographicimage that results from a laser illumination of the DMD array of FIG.13B. A bright spot due to the zero order energy, analogous to a DCcomponent of an electronic signal, appears positioned at the center ofthe image. This zero order component will exist for each diffractionpattern, because the DMD in this arrangement modulates light intensity(amplitude modulation) and not phase. The first order component, whichreproduces the desired image, is shown at the upper left of FIG. 13C.The resulting image also has a second first order component image, aconjugate image, formed at the lower right portion of FIG. 13C. Theconjugate first order image is flipped about the zero order spot. Eachimage formed using a diffraction pattern will also have a conjugateimage and a zero order spot.

As shown by the diffractive pattern in FIG. 13B, the diffraction orhologram imaging system is not projecting an image through the DMD arraysuch as would be the case for a DMD in a video system using opticalprojection. The pattern at the far field image plane and the diffractionpattern displayed using the DMD relate mathematically by a Fouriertransform. When illuminated by a coherent light source, the diffractionpattern produces wavefronts that interfere constructively anddestructively corresponding to the diffracted light. The desired imageappears at a plane some distance from the DMD. In example arrangements,a variety of diffraction patterns can display on a DMD mirror array in asequence to form arbitrary and desired scan patterns at some distance.

The diffraction pattern and the resulting image at the far field planeare related by a two-dimensional Fourier transform, so algorithms forgenerating these diffractive patterns can use Fourier transforms inexample arrangements. Algorithms for generating these patterns fordisplay on the DMD are somewhat more complicated because the DMD is abinary, amplitude only modulator.

Some fast algorithms for generating diffractive patterns or hologramsfor display on a DMD are described in a paper entitled “Fast algorithmsfor generating binary holograms,” authored by Stuart et al.,arXiv:1409.1841[physics.optics], 5 Sep. 2014, (hereinafter, “Stuart et.al.”) which is hereby incorporated by reference herein in its entirety.In Stuart et. al., the fast algorithms include an ordered ditheringalgorithm and a weighted Gerchberg-Saxton algorithm. Examplearrangements can use additional algorithms to develop diffractionpatterns. An example algorithm includes identifying a far field imagepattern to be created in the field of view; zero padding the imagepattern; and taking the inverse Fourier transform of the zero paddedpattern using a fast Fourier transform. The method continues byquantizing the resulting complex inverse fast Fourier transform (IFFT)data to get a binary pattern for display using the DMD, and sub samplingthe binary pattern to arrange it for the particular DMD mirrororientation. By simulating the far field image using FFTs, and observingthe resulting far field image, recursive improvements can adjust thediffraction pattern until the desired far field image results. Forexample, these recursive improvements can compensate for device specificvariations in die flatness, such as to obtain the correct far fieldimage without modifying the DMD. In example arrangements, distortioneffects such as astigmatism, geometric distortion, and distortionresulting from the physical characteristics of optical elements can becompensated by further modifying the diffraction patterns displayed onthe DMD. Distortion can be reduced or eliminated in the scan beam ofexample arrangements by applying correction to the diffraction patterns,as further described hereinbelow.

FIG. 14 shows a block diagram illustrating one algorithm for generatinga DMD binary, amplitude-only diffraction pattern. FIG. 14 shows adesired pattern for a far field image, “Pattern Image” 1401. One simplemethod of creating a DMD pattern that produces the desired far fieldimage 1401 is to apply an inverse fast Fourier transform (IFFT) to theimage. To compute the IFFT efficiently, various computing techniquessuch as a discrete fast Fourier transforms, or “DFFT” are useful.Processors optimized for DFFT computations, such as co-processors,digital signal processors, and vector processors are useful to computethe inverse DFFT. As shown in FIG. 14, the result is a two-dimensionalarray labeled “Complex IFFT Image” 1403. As shown in FIG. 14, theComplex IFFT Image 1403 has no obvious relationship to the Pattern Image1401. The Complex IFFT image includes components that are not of binaryvalues. To form a corresponding diffractive pattern for display usingthe DMD, which is a binary amplitude modulator with the binary states ONand OFF, the system performs additional processing. This processing caninclude quantization or binarization of the Complex IFFT Image 1403 toallow display on the binary DMD. Several methods create a binarydiffractive image that produces a desired far field image, such asmethods described by Stuart et. al., described hereinabove. Also, thesystem maps the quantized diffraction pattern data to match the data tothe selected orientation type of DMD. If the DMD is a square pixel or“Manhattan” mirror orientation, the system performs one type of mapping.If the DMD is a diamond pixel orientation DMD, the system performs adifferent mapping to map that data onto the DMD. In examplearrangements, the methods compute a diffraction pattern for displayusing the DMD that will produce the desired far field image.

Iterative optimization steps can better match the far field image to thedesired image. In an example, the Gerchberg-Saxton algorithm is usefulas an iterative algorithm. FIG. 14 illustrates the optimization processby the “Iterative Optimization” path 1407. This iterative processcontinues for each desired pattern to obtain a corresponding diffractivepattern for display using the DMD. In example arrangements, distortionor aberrations due to geometric distortion or optical distortion iscompensated by modifying the diffraction pattern.

Because the diffraction pattern is a two-dimensional data array fordisplay using the DMD, the diffraction patterns can be stored in memoryas diffraction pattern templates. Additional patterns can be stored inmemory in a system for retrieval and display. The processing to computethe diffraction patterns using the inverse Fourier transform can beperformed “offline” or during a system calibration process, and examplearrangements do not require a system to compute these diffractionpatterns in real time or in the field. However, in an alternativeexample, real time processing can compute the diffractive patterns; thisapproach avoids storing all of the possible diffractive patterns neededin a memory. In an example arrangement, a two dimensional polynomial canbe computed in a real time operation to perform the correction to thediffraction pattern to compensate for distortion, as further describedhereinbelow.

FIG. 15 depicts in a simple circuit block diagram an arrangement 1500. Amicroprocessor, mixed signal processor, digital signal processor,microcontroller or other programmable processor device 1511 executesinstructions that cause it to output digital video signals DVO fordisplay by the DMD. A variety of sources may provide the digital videosignals labeled DVI in FIG. 15. In example arrangements, a system canperform the inverse Fourier transforms described hereinabove to producethe DVI data needed for diffractive patterns in real time. In analternative arrangement, the DVI data can come from stored diffractionpattern templates computed before operation of the system, or fromdiffraction patterns stored in a calibration operation duringmanufacture of the system. FIG. 15 shows an optional diffraction patternmemory 1513 for storing diffraction patterns coupled to processor 1511.Dynamic memory (DRAM), static random access memory (SRAM), non-volatileread write memory such as EEPROM, FLASH, EPROM and other data memorytypes can be used to store the diffraction patterns. The processor 1511couples to a digital DMD controller circuit 1503. Digital DMD controllercircuit 1503 is another digital video processing integrated circuit. Inan example, digital DMD controller circuit 1503 is a customizedintegrated circuit or an application specific integrated circuit (ASIC).FIG. 15 also shows an analog circuit that manages power and LEDillumination referred to as the “power management integrated circuit”(PMIC DMD Controller) 1515. PMIC 1515 controls the intensity and powerto the coherent light source laser 1509. The Digital DMD Controller 1503provides digital data to the DMD 1501 for modulating the illuminationlight that strikes the DMD surface. PMIC 1515 provides power and analogsignals to the DMD 1501. The light rays from the illumination source1509 travel to illumination components in block 1514. As describedhereinabove, the illumination components in block 1514 includeconverging optics to focus rays onto the surface of the DMD 1501. Thelight strikes the reflective mirrors inside DMD 1501. The reflectedlight for projection leaves the surface of the DMD 1501 and travels intothe negative optics 1507. In an arrangement described hereinabove,negative optics 1507 includes part of a reverse refractive telescopeoptical element that operates to transmit the diffracted light in anexpanded scan angle. Together the integrated circuits 1511, 1503 and1515 cause the DMD 1501 and the optical components 1514, 1507 to outputthe diffracted light.

Example integrated circuits that can implement the circuit shown in FIG.15 include DMD controller ICs from Texas Instruments Incorporated.Example DMD controller ICs include the DLPC3430 DMD controller and theDLPC2601 ASIC device that can perform both digital and analog controllerfunctions. Analog DMD controller devices from Texas InstrumentsIncorporated include the DLPA2000 device. Laser controller devices canpower on and off the laser 1509 or form pulses.

The DMD of FIG. 15 can be a DMD device from Texas InstrumentsIncorporated such as the DLP2010DMD, which is a 0.2-inch diagonal devicethat provides wide VGA (WVGA) resolution. Example arrangements caninclude many other DMD devices that are available from Texas InstrumentsIncorporated.

FIG. 16 depicts in a top view a block diagram for an example system1600. In FIG. 16, a laser or other illumination source 1603 illuminatesan array of micromirrors on DMD 1607. The angle of the beam from thelaser to the DMD is determined by the tilt angles of the DMD chosen forthe arrangement and by the desired path leaving the DMD surface. The DMD1607 can be any DMD device. In more alternative arrangements, the DMD1607 can be another reflective SLM, including a PSLM and a reflectiveLCoS device as described hereinabove.

In FIG. 16, a scan beam exits a negative optical element 1601 in threepositions: 1611, 1613, and 1619. The beam positions 1611 and 1619illustrate the scan angle θ obtained. In the example shown in FIG. 16,the negative optical element 1601 is a part of a reverse refractivetelescope. A processor 1635 provides data “DMD Data” to the DMD 1607 andcontrols the laser 1603. The processor 1635 can include multiple customor commercially available integrated circuits as described hereinaboveto control the data displayed using the DMD and the laser pulses thatilluminate the DMD. An optional storage for diffraction patterns couplesto the processor 1635, the “Diffraction pattern memory” 1637, and storestwo-dimensional arrays for displaying diffraction patterns using theDMD.

For example, as described hereinabove, the diffraction patterns arecorrected from an initial value to compensate for distortion oraberrations in the scan beam caused by geometric distortion in the DMDdevice, or by other distortion caused by optical distortion.

Correction of a two dimensional diffraction pattern can be described bya wavefront correction factor φ. If the distortion is in one dimension,a correction factor φ can be one-dimensional. In a two-dimensionalcorrection, the wavefront correction can be approximated using an n-thorder polynomial expressions fitted to the optical path difference dataor to a theoretical model for the system. The correction factor φ iscomputed in real time, or alternatively, the stored diffraction patternsis modified and the corrected diffraction patterns can be stored forlookup and display. In one arrangement, the correction is done in anoff-line computation; while in an alternative arrangement the correctioncan be determined using modeling or simulation.

The two dimensional array of corrected values for positions x and y canbe described by the following Equation (1):

H(x,y)=A exp(jφ(x,y))  (1)

-   -   where H(x, y) is the corrected diffraction pattern in two        dimensions, and    -   jφ(x, y) is a correction factor in two dimensions for each        micromirror at a position x, y.

In an example arrangement, real time processing of the polynomial ofEquation (1) computes the diffraction patterns with correction in realtime. Alternatively, the algorithm is performed “off-line” with theresults stored in a corrected diffraction pattern memory. For example,the computed corrected diffraction patterns can be stored as atwo-dimensional lookup table that is addressed by the mirror index x,yfor each position in the DMD array.

FIG. 17 depicts a flow diagram for an example method 1700. In FIG. 17,the method begins at step 1701, where a desired image pattern isdetermined. This image will appear in the field of view. At step 1703,the method provides illumination from a source. In step 1705, a positiveoptical element receives and converges the light rays. At step 1707, theconverged light rays fall onto the surface of a reflective element. Asdescribed hereinabove, the reflective element can be an analog MEMSmirror, a DMD, a PSLM, or an LCoS device. At step 1709, the rays reflectinto a negative optical element. As described hereinabove, in examplearrangements the negative optical element is part of a reverserefractive telescope. In alternative arrangements, the negative opticalelement can be part of an afocal lens or lenses. At step 1711, the beamformed by an illumination source and the reflective element scans acrossa field of view. Because the beam exits the system through the negativeoptical element, the scan angle for the beam is increased.

FIG. 18 illustrates another method arrangement 1800. At a step 1801, adesired image pattern is determined. At step 1803, the method performsan inverse Fourier transform. Fast Fourier transforms such as discretefast Fourier transforms (DFFTs) can perform the inverse Fouriertransform. For example, at step 1805, the method corrects the inverseFourier transform, using a two dimensional polynomial such as Equation(1). At step 1806, the method performs a quantization or binarizationstep. Because the DMD is a binary amplitude modulator with two states,ON and OFF, the method quantizes the corrected inverse Fourier transformfor use with the binary format of the DMD. In alternative examplemethods, the steps of FIG. 18 can be in another order. However, in theexample of FIG. 18, the correction is applied before the quantization.As described hereinabove, the correction can compensate the beampatterns for geometric distortion inherent when using a DMD to reflectlight, or for other distortion due to optical characteristics of thesystem. In step 1807, the method subsamples the quantized correcteddiffraction pattern formed in step 1806 to match the particular DMDmirror orientation in the system. DMD devices differ according toorientation of their micromirrors, such as in either square or diamondorientation. For a diamond pixel DMD device, a different subsamplingapplies than that for a square pixel DMD device. At step 1809, thediffraction pattern displays on the DMD. At step 1811, the methodilluminates the diffraction pattern on the DMD by the light source toform the image pattern. The image pattern forms as wavefronts ofdiffracted light that constructively and destructively interfere as thewavefronts move away from the DMD, and the desired pattern appears inthe field of view. Step 1813 shows an optional storage step. Diffractionpatterns can be stored in a pattern memory for later retrieval anddisplay. Alternatively, the method can compute the DMD diffractionpatterns as needed in real time, and the correction of step 1805 can becomputed in real time, such as using Equation (1). Algorithms also existthat generate periodic diffraction patterns and that can be performedquickly without the use of Fourier transforms, and these algorithms canbe used with example arrangements.

FIG. 19 illustrates in a flow diagram a method 1900 for formingdiffraction pattern templates for use in example arrangements. In FIG.19, the method begins at step 1901 where a desired scan pattern isdetermined. For example, the method can select a raster scan pattern. Atstep 1903, for each image in the pattern, the method performs an inverseFourier transform. Because a scan pattern is a sequence of images, themethod performs a plurality of inverse Fourier transforms. At step 1905,correction is applied. The correction can use a two-dimensionalpolynomial, such as in Equation (1). At step 1907, the method quantizesor performs binarization for each of the inverse Fourier transforms toform a diffraction pattern sequence for the binary DMD array. At step1909, each of the corrected quantized diffraction patterns is subsampledto map it to the DMD device used in a particular arrangement. Thesubsampled and quantized corrected diffraction pattern sequence is thenstored in memory for use.

The method of FIG. 19 illustrates that the diffraction patterns can becomputed “off-line” or in a calibration operation during manufacture ofan example system, and then the patterns can be stored for later use. Inthis approach, the system does not have to perform real timecomputations of the diffraction patterns during operation.

FIG. 20 illustrates in a flow diagram a method 2000 for using the storeddiffraction patterns to form a scan pattern. Beginning at step 2001, themethod selects a desired scan pattern from a number of stored scanpatterns. At step 2003, the method retrieves the stored correcteddiffraction patterns corresponding to the desired scan pattern. At step2005, a looping operation begins. For each diffraction pattern in asequence needed to form the selected scan pattern in the field of view,the method displays the selected diffraction pattern using the DMD. Atstep 2007, the method illuminates the DMD to form an image that is partof the scan pattern. At step 2009, the method updates the pattern of theDMD to form a scan beam in the field of view. At step 2011, the scanbeam reflects from the surface of the DMD into a negative opticalelement to expand the scan angle of the beam. The method continueslooping through the sequence to continue scanning the field of view byreturning to step 2005.

The example arrangements form beams for use in lidar systems useful in awide variety of applications. Mobile navigation and collision avoidancesystems, robotics, autonomous vehicle control, security, industrialautomation, surveying, mapping, and meteorology are all applications forlidar systems including example arrangements. The systems usesolid-state components without the need for mechanical parts. BecauseDMD devices can operate even with a large percentage of failedmicromirrors, the systems using the DMD arrangements are inherentlyrobust and reliable and are relatively low in cost. Similarly, theanalog MEMS mirror, DMD PSLM and LCoS arrangements are also robust andreliable and relatively low in cost, with solid-state components. Use ofa single illumination source and the lack of motors and rotors furtherreduces system cost, reduces system maintenance requirements, andincreases reliability over conventional approaches. The ability tocorrect for distortion further enhances the system performance withoutadditional cost or added components.

Modifications are possible in the described arrangements, and otherarrangements that form additional aspects of this application arepossible that are within the scope of the appended claims.

What is claimed is:
 1. A method comprising: obtaining, by at least oneprocessor, an image pattern sequence; computing, by the at least oneprocessor, a sequence of diffractive images corresponding to the imagepattern sequence; correcting, by the at least one processor, thesequence of diffractive images to compensate for distortion by a spatiallight modulator, to produce a sequence of corrected diffractive images;mapping, by the at least one processor, the sequence of correcteddiffractive images to produce a mapped sequence of diffraction patterns;setting, by the spatial light modulator, elements of the spatial lightmodulator to the mapped sequence of diffraction patterns; and for themapped sequence of quantized diffraction patterns displayed using thespatial light modulator, illuminating, by a light source, the spatiallight modulator to produce the image pattern sequence.
 2. The method ofclaim 1, wherein computing the sequence of diffractive imagescorresponding to the image pattern sequence comprises applying aninverse Fourier transform to image patterns in the image patternsequence.
 3. The method of claim 1, further comprising: storing thesequence of corrected diffractive images in a memory.
 4. The method ofclaim 3, further comprising: retrieving the sequence of correcteddiffractive images from the memory; and using the spatial lightmodulator, displaying the retrieved sequence of corrected diffractiveimages.
 5. The method of claim 1, wherein correcting the sequence ofdiffractive images further comprises computing a two dimensionalpolynomial including a correction factor.
 6. The method of claim 5,wherein computing the two dimensional polynomial includes computing:H(x,y)=A exp(jφ(x,y)), where H(x, y) is the corrected diffractive imagesin two dimensions, and jφ(x, y) is a correction factor in two dimensionsfor each element of the spatial light modulator at a position x, y. 7.The method of claim 1, wherein the spatial light modulator is a digitalmicromirror device, a phase light modulator, or a liquid crystal onsilicon device.
 8. The method of claim 1, further comprising quantizingthe sequence of corrected diffractive images.
 9. A method comprising:determining, by at least one processor, an image pattern sequence;computing, by the at least one processor, a sequence of diffractiveimages corresponding to the image pattern sequence; correcting, by theat least one processor, the sequence of diffractive images to compensatefor distortion by a spatial light modulator, to produce a sequence ofcorrected diffractive images; mapping, by the at least one processor,the sequence of corrected diffractive images to produce a mappedsequence of diffraction patterns; and storing, in memory, the mappedsequence of diffraction images.
 10. The method of claim 9, whereincomputing the sequence of diffractive images corresponding to the imagepattern sequence comprises applying an inverse Fourier transform toimage patterns in the image pattern sequence.
 11. The method of claim 9,further comprising quantizing the sequence of corrected diffractiveimages.
 12. The method of claim 9, wherein correcting the sequence ofdiffractive images further comprises computing a two dimensionalpolynomial including a correction factor.
 13. The method of claim 12,wherein computing the two dimensional polynomial includes computing:H(x,y)=A exp(jφ(x,y)), where H(x, y) is the corrected diffractive imagesin two dimensions, and jφ(x, y) is a correction factor in two dimensionsfor each element of the spatial light modulator at a position x, y. 14.A system comprising: a spatial light modulator; a light source opticallycoupled to the spatial light modulator; and at least one processorcoupled to the spatial light modulator, the at least one processorconfigured to: obtain an image pattern sequence; compute a sequence ofdiffractive images corresponding to the image pattern sequence; correctthe sequence of diffractive images to compensate for distortion by aspatial light modulator, to produce a sequence of corrected diffractiveimages; map the sequence of corrected diffractive images to produce amapped sequence of diffraction patterns; and instruct the spatial lightmodulator to display the mapped sequence of diffraction patterns;wherein the spatial light modulator is configured to set elements basedon the mapped sequence of diffraction patterns; and wherein the lightsource is configured to illuminate the spatial light modulator toproduce the image pattern sequence.
 15. The system of claim 14, whereincomputing the sequence of diffractive images corresponding to the imagepattern sequence comprises applying an inverse Fourier transform toimage patterns in the image pattern sequence.
 16. The system of claim14, wherein correcting the sequence of diffractive images furthercomprises computing a two dimensional polynomial including a correctionfactor.
 17. The system of claim 16, wherein computing the twodimensional polynomial includes computing:H(x,y)=A exp(jφ(x,y)), where H(x, y) is the corrected diffractive imagesin two dimensions, and jφ(x, y) is a correction factor in two dimensionsfor each element of the spatial light modulator at a position x, y. 18.The system of claim 14, wherein the spatial light modulator is a digitalmicromirror device, a phase light modulator, or a liquid crystal onsilicon device.
 19. The system of claim 14, wherein the at least oneprocessor is further configured to quantize the sequence of correcteddiffractive images.
 20. The system of claim 14, further comprisingmemory coupled to the at least one processor, wherein obtaining theimage pattern sequence comprises retrieving the image pattern sequencefrom the memory.